Conventionally, in order to know the wavelength of a laser beam, the laser beam irradiated from a laser beam source 1 is processed by a preprocessing system 2 including a lens and a filter so as to be suitable for a spectroscope 3, as shown in FIG. 8. The processed beam is then guided into an incident slit 3a in the spectroscope 3 and introduced in the direction determined by diffraction gratings 4b and 3c (or prisms) and a rotary stage in the spectroscope 3. The beam exiting the output slit 3d is then sensed to a signal processing system 4 to sense the wavelength of he laser beam from the angle of the rotary stage (the relationship between the angle of the rotary stage and the wavelength is known in advance).
Therefore, where the wavelength of the laser beam is controlled to a predetermined value, the angle of the rotary stage is adjusted in advance such that when a laser beam having a predetermined wavelength is entered, the beam is irradiated from the outlet slit 3d, and the wavelength of the laser beam is identified by a wavelength selective means (for example, an adjustable etalon) such that the laser beam is irradiated from the outlet slit 3d.
The identification of the wavelength using the spectroscope is, however, necessarily accompanied by troublesome alignment. In addition, in order to obtain a good result of analysis, a large-sized spectroscope is needed to thereby increase the entire size and cost of the system.
This invention derives from the contemplation of these situations. The object of this invention is to provide an apparatus which is capable of sensing and controlling the wavelength of a laser beam without using a spectroscope and a method of controlling the wavelength of the laser beam.